1. Field
The present application relates to systems and methods for anatomical and functional matching and imaging of tissue characteristics.
2. Background Art
Electrical mapping of the heart has emerged as an important tool for treatment monitoring of arrhythmias such as ventricular tachycardia. The map of local timings of electrical activation in the ventricle can identify abnormally conducting regions and guide radio frequency (RF) ablation treatments. Currently available clinical mapping systems are, however, invasive, since they require catheterization for introduction into the heart chamber. Therefore, electrical mapping cannot be performed for early detection of diseases or follow-up of chronic diseases such as heart failure.
Electromechanical Wave Imaging (EWI) has recently been introduced as a non-invasive, inexpensive, ultrasound-based modality, which can potentially map the electrical activation of the heart transmurally along various echocardiographic planes. This imaging modality is based on the measurement of small, transient deformations occurring in the myocardium a few milliseconds after, but following similar patterns as, the electrical activation. More specifically, after the action potentials reach the myocytes, the latter undergo depolarization followed by an uptake of calcium, which triggers contraction a few milliseconds later. Therefore, by measuring the onset of this contraction, the activation pattern across the entire myocardium can be mapped.
Over the past two decades, several methods have been developed for measuring deformations using ultrasound-based methods. Two-dimensional speckle-tracking-based motion estimation techniques have been implemented on clinical systems. Different approaches based on B-Mode or radio-frequency (RF) speckle tracking, or phase-tracking techniques have also been proposed in the literature for myocardial contractility assessment. Recently, open architecture ultrasound systems enabled motion estimation at very high effective RF-frame rates of standard echocardiographic views. The full view of the heart is divided into five to seven sectors acquired at very high frame rates and a full view ciné-loop is then reconstructed via electrocardiogram(ECG)-gating. Such high frame rates and the RF phase information increase the estimation quality and thus the reliability of two-dimensional displacement and strain mapping.
The increase in the frame rate did not only allow a better precision in the RF-based motion estimation, but also achieved a temporal resolution on the same time scale as that of the electrical propagation. More specifically, it allowed the detection of transient phenomena that occur during both isovolumic phases. For example, it was possible to identify the mechanical waves in the myocardial wall occurring when the valves open and close. Incremental displacements waves generated by the early contraction of myocytes, i.e., the Electromechanical Wave (EMW), have been depicted on EWI ciné-loop and images, and their correlation with the electrical activation velocity and pacing scheme have been verified. More recently, the EMW was reproduced in simulations and shown to be correlated with simulated and experimental electrical activation patterns.
Alternative methods for assessment of local electrical properties in vivo involve the use of electrode arrays, either by mounting an electrode sock around the heart through open-heart surgery to map the epicardial activation or by using electrode catheters. Newly developed non-invasive techniques based on heart models provided fully three-dimensional activation sequences. A method based on magnetic resonance (MR) tagging has also been proposed, where the subepicardial contraction sequence was mapped and compared to the electrical activation maps obtained with an epicardial electrode sock.
ECG-gating methods are common in biomedical imaging technologies to achieve frame-rate that are sufficient to obtain an accurate depiction of the cardiac motion either in two or three dimensions. It is useful, for instance, when imaging the heart or the cardiovascular system using computed tomography, magnetic resonance imaging, or nuclear imaging.
Though previous efforts have obtained high frame rates using ECG gating, for the analysis of diseases such as ventricular tachycardia, the ECG may not be regular. For example, when atrio-ventricular dissociation occurs, the atria and ventricles follow different rhythms, which may compromise the use of the ECG for co-registration of adjacent sectors. Accordingly, there is a need in the art for a non-invasive imaging technique that is not reliant on independent measurements of the electrical activity of the subject tissue.